1. Field of the Invention
The present invention relates to a process for producing an .alpha.-Sialon powder, more specifically, to a process for producing a fine .alpha.-Sialon powder with a narrow particle size distribution, and thus having an excellent sinterability.
2. Description of the Related Art
.alpha.-Sialon is a solid solution type substance in which aluminum solute is on the silicon sites, oxygen solute is on the nitrogen sites, and one or more other metal solutes are in the interstices of the lattice, of .alpha.-silicon nitride. .alpha.-Sialon is represented by the general formula: EQU M.sub.x (Si, Al).sub.12 (O, N).sub.16
where M is a metal which is a solute incorporated in the interstices of the lattice and x has a value larger than 0 but not larger than 2.
.alpha.-Sialon has many characteristics required for engineering ceramics, such as high strength, high hardness, low expansion coefficient, excellent corrosion resistance and the like. Further, it has excellent electrical characteristics due to the metal solute incorporated in the interstices of the .alpha.-Sialon lattice, and accordingly, is promising for many electronic ceramic applications. Furthermore because it principally does not have a grain boundary phase, it has a light permeability.
It is known to produce a sintered .alpha.-Sialon by hot-pressing a mixture of, for example, crystalline silicon nitride, aluminum nitride, and an oxide of a metal which may be incorporated in the interstices of .alpha.-Sialon lattice, or by heating mixture as mentioned-above in a nitrogen gas atmosphere at normal pressure or with pressure. In these processes, the formation of .alpha.-Sialon phase and sintering proceed simultaneously, resultig in nonuniformity of the microstructure of the .alpha.-Sialon and elemental distribution therein, which tends to cause defects.
In order to solve the above problems, synthesis of an .alpha.-Sialon powder having an .alpha.-Sialon phase is under study for use as a raw material for sintered .alpha.-Sialon.
It is known to synthesize an .alpha.-Sialon powder by adding an organic solvent to a mixed powder of a silicon alkoxide, an aluminum alkoxide, and yttrium nitrate or a calcium alkoxide, dispersing a carbon powder therein, hydrolyzing the dispersion to form a precipitate, drying the precipitate, and calcinating the dried precipitate in a nitrogen gas atmosphere (Abstract of 24th Symposium on Basic Ceramics). This process can provide an .alpha.-Sialon powder having a fairly high purity but has drawbacks such as unreacted carbon remaining in the .alpha.-Sialon powder and the expense of the raw materials used. Hence, the process is industrially not satisfactory from the standpoints of raw materials and process per se.